An avalanche photodiode (APD) is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. APDs can be thought of as photodetectors that provide a built-in first stage of gain through avalanche multiplication. From a functional standpoint, they can be regarded as the semiconductor analog to photomultipliers. By applying a high reverse bias voltage (typically 100-200 V in silicon), APDs show an internal current gain effect (often around 100×) due to impact ionization (avalanche effect).
The internal gain of APDs can provide higher sensitivity than p-i-n photodiodes, which is beneficial for many optical communication and sensing applications. However, as discussed above, the origin of the APD gain is impact ionization, a stochastic process that results in excess noise and limits the gain-bandwidth product. The noise power spectral density of an APD, φ, is given by the expression φ=2q·I·M2·F(M)·R(ω) where q is the charge on an electron, I is the current, M is the avalanche gain, F(M) is the excess noise factor caused by the random nature of the multiplication process, and R(ω) is the device impedance. For the past four decades, reducing the excess noise factor, F(M), has been a focus of APD research and development. One structure that was proposed to achieve very low noise is referred to as the “staircase APD.” The band structure diagrams of a two-step staircase avalanche photodiode at zero bias and under reverse bias are illustrated in FIGS. 1A and 1B, respectively, in accordance with an embodiment of the present invention. Referring to FIGS. 1A and 1B, unlike conventional APDs, in which impact ionization occurs relatively uniformly throughout the entire multiplication region, in the staircase structure, avalanche events occur proximate to the sharp bandgap discontinuity. As electrons in the wide bandgap region (Eg2) move into the narrow bandgap region (Eg1), their excess energy enables immediate impact ionization. These discontinuities function somewhat like dynodes in a photomultiplier in which the gain position is localized. As a result, the gain process is more deterministic, with concomitant reduction in gain fluctuations, and, thus, lower excess noise. Unfortunately, initial studies of staircase APDs used the AlxGa1-xAs material system, which has inadequate band offsets and the projected noise characteristics were never achieved. That is, current staircase APDs exhibit indirect bandgaps, small conduction band offsets relative to the smallest achievable bandgap and large valence band offsets.